Sentinel animals are an essential component of animal health surveillance programs, providing the primary means of detecting adventitious agents in laboratory animal colonies. An optimal program attempts to maximize exposure of the sentinel animals to infectious agents and to minimize the time required for detection. Our study compared the classical aerosol exposure method with a technique utilizing sensitive strains of mice exposed to both aerosols and soiled bedding from the research colony. Eight cages of mice containing 12 mice (3 each of 4 different strains) per cage were housed without filter bonnets on the bottom shelf of 4 out of 12 racks in an animal room which had a history of sporadic mouse hepatitis virus (MHV) infections. Half of the cages received a composite sample of bedding used previously be experimental mice in the room and the other half received fresh unused bedding. The sentinel mice were bled at monthly intervals for MHV serology and observed twice weekly for clinical changes. After 5 months, all of the mice in cages receiving used bedding had seroconverted to MHV and three of the groups were positive for MHV and all were negative for mites. In addition,2 of the groups of mice receiving seroconverted 3 weeks before any of the groups receiving fresh bedding. These finding indicate the importance of exposing sentinel mice to used bedding to enhance transmission of MHV and mites. This study has been accepted for publication in the July 1989 issue of Laboratory Animal Science.